Synergistic catalysis of hybrid nano-structure Pd catalyst for highly efficient catalytic selective hydrogenation of benzaldehyde

Article abstract of DOI:10.1016/j.cattod.2020.01.022

Selective hydrogenation of benzaldehyde is a green and sustainable technology to produce benzyl alcohol. Herein, we report a hybrid nano-structure catalyst(Pd/@-ZrO₂/AC) by photochemical route for selective hydrogenation of benzaldehyde under m

Full text of DOI:10.1016/j.cattod.2020.01.022

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Synergistic catalysis of hybrid nano-structure Pd catalyst for highly efficient
catalytic selective hydrogenation of benzaldehyde
Yanji Zhanga,b, Jicheng Zhoua,b,*, Kai Lia,b, Mengdie Lva,b
a Key Laboratory of Green Catalysis and Chemical Reaction Engineering of Hunan Province, Xiangtan, 411105, Hunan Province, China
b School of Chemical Engineering, Xiangtan University, Xiangtan, 411105, Hunan Province, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Synergistic catalysis
Hybrid nano-structure catalyst
Benzaldehyde hydrogenation
Photochemical route
Selective hydrogenation
Selective hydrogenation of benzaldehyde is a green and sustainable technology to produce benzyl alcohol.
Herein, we report a hybrid nano-structure catalyst(Pd/@-ZrO2/AC) by photochemical route for selective hy-
drogenation of benzaldehyde under mild reaction conditions. The Pd/@-ZrO2/AC catalyst exhibited outstanding
activity(100%) and high selectivity for benzyl alcohol (> 98 %), much higher than that of Pd/ZrO2 and Pd/AC.
The high catalytic activity was attributed to the special structure and the synergistic effect. A series of Pd/@
-MexOy/AC(MexOy: TiO2, CeO2, La2O3) catalysts further confirmed the synergistic effect in this hybrid nano-
structure for this reaction. This work clearly demonstrates that loading nano-metal on high specific surface area
materials with nano-semiconductors can increase the catalytic activity and open up a new direction for the
synergistic catalysis of hybrid nano-structure catalysts.
1. Introduction
in this reaction, such as properties of support, metal-support interac-
tion, crystallite size of metals and so on. The choice of appropriate
Benzyl alcohol is an important intermediate in the synthesis of a
wide variety of esters, which is used in vitamins, pharmaceuticals,
pesticides, flavor, fragrances, and various esters [1–4]. Previously, the
conventional synthetic method for obtaining the corresponding alcohol
was to hydrolyze benzyl chloride using sodium hydroxide, but this re-
action released chlorine produced significant by-products (benzyl ether
and NaCl), which were harmful to the application and caused serious
environmental toxicity [5]. Recently, it has been found that the selec-
tive hydrogenation of benzaldehyde to benzyl alcohol is a better choice
support can have a significant effect on both the activity and selectivity
in certain reactions. Various researchers compared the activity of Pd
with different supports. The most commonly used supports were carbon
material and metal oxide. Xinhe Bao et al. [24] studied the selectivity of
benzyl alcohol by using the functionalization of carbon nanotubes
(CNTs) as the support of Pd. The introduction of oxygen-containing
groups in the CNTs reduced the selectivity to the intermediate benzyl
alcohol (27 %), whereas doping with N atoms significantly inhibited the
further hydrogenation of benzyl alcohol and thus increased its se-
lectivity (96 %). GeonJoong Kim et al. [25] prepared Pd/carbon na-
notubes (CNTs), Pd/r-Al2O3 and Pd/SiO2 catalysts. The order of se-
lectivity to benzyl alcohol was Pd/CNT (4.7 %) < Pd/SiO2 (88.4
%) < Pd/γ-Al2O3 (92.9 %), and the corresponding benzaldehyde con-
version was Pd/γ-Al2O3 (15.5%) < Pd/SiO2 (70.3%) < Pd/CNTs
(1.0%). Carmine Capacchione et al. [26] compared the activity of Pd/
mayenite(Ca12Al14O33) and Pd/C catalysts, because of the presence of
hydride species on the Ca12Al14O33, the catalytic performance of Pd/
Ca12Al14O33 was superior in terms of selectivity at 120℃ (91 % con-
version and 99 % selectivity to benzyl alcohol within 5 h). But low
catalyst activity of Pd/Ca12Al14O33 at room temperature (68 % con-
version and 99 % selectivity to benzyl alcohol within 5 h). Masaki
Okamoto et al. [27] reported the use of polyethylene glycol (PEG) as a
[6–8]. However, it is still
a challenge to avoid further hydro-
deoxygenation of benzyl alcohol into toluene. The preparation path-
ways are shown in Scheme 1.
Various metal catalysts such as Ni [9–11], Cu [12], Co [13], Pt
[11,14–17], Au [5,18,19], Ru [20–22] and Pd [3,7,8,11,23–27] were
used for the hydrogenation of benzaldehyde to benzyl alcohol. Palla-
dium was the most widely used catalyst, but it was a challenge to
achieve satisfactory selectivity of benzyl alcohol when benzaldehyde
was completely reacted under mild conditions. Therefore, increasing
the activity of palladium catalysts while maintaining their high se-
lectivity has become an urgent task for further development in this
field. In the hydrogenation of aromatic ring compounds, the activation
of hydrogen is an important step. Many factors play an important role
⁎ Corresponding author at: School of Chemical Engineering, Xiangtan University, Xiangtan, 411105, Hunan Province, China.
E-mail addresses: zhoujicheng2012@126.com, zhoujicheng@sohu.com (J. Zhou).
https://doi.org/10.1016/j.cattod.2020.01.022
Received 5 March 2019; Received in revised form 16 October 2019; Accepted 19 January 2020
0920-5861/©2020ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:YanjiZhang,etal.,CatalysisToday,https://doi.org/10.1016/j.cattod.2020.01.022

Y. Zhang, et al.
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calcination at 500℃ for 4 h. The preparation of ZrO2/CNTs was the
same as above, the only difference was to ZrO2/AC with ZrO2/CNTs.
Preparation of TiO2/AC. 3 g tetrabutyl titanate was first dissolved in
25 mL ethanol by stirring and then 1.3 mL acetic acid was added into
the mixture to obtain solution A. A solution of 15 mL ethanol and
1.5 mL distilled water was denoted by solution B. Then solution B was
added into solution A dropwise under vigorous stirring to form sol C.
Subsequently, the pretreatment AC was dispersed into sol C and nitric
acid was used to adjust pH to 3. After being treated by ultrasonic agi-
tation, the mixture was left at ambient temperature to form the gel. The
obtained material was dried at 80℃ for 12 h, and then calcined at 550℃
for 2 h.
Preparation of CeO2/AC. Ce(NO3)3·6H2O(1.46 g) dissolved in
ethanol(120 mL), while stirring, add the pre-processed AC(5 g) in it,
1 mol/L NaOH was added dropwise until the pH of the resulting solu-
tion was 10, put the solution in the microwave with the power was set
214 W for 10 min(on for 10 s, off for 20 s). After cooled down to room
temperature, washed with distilled water and ethanol then left to dry at
120℃ for 12 h.
Preparation of La2O3/AC. In a typical synthesis, La(NO3)2·nH2O
(0.554 g) dissolved in distilled water(120 mL), while stirring, add the
pre-processed AC(5 g) in it, 0.5 mol/L NH4HCO3 was added dropwise
until the pH of the resulting solution was 7.0, put the solution in the
microwave with the power was set 400 W for 16 min(on for 30 s, off for
30 s). After filtrating, washing and dry at 70℃ for 4 h, finally achieved
composite support La2O3/AC after calcined at 550℃ for 2 h.
According to principles of the spontaneous monolayer distribution
[32], when the content of metal oxide is less than a certain threshold
and surface area of the support is large enough, metal oxide would
disperse on the support at the state of the spontaneous monolayer
distribution, so an appropriate amount of metal oxide can form a single
or multi-layer nano-semiconductor layer on AC(CNTs).
Scheme 1. The preparation pathways of benzyl alcohol by hydrolyze benzyl
chloride(a) and benzaldehyde hydrogenation(b).
modifiers for Pd/SiO2 catalyst in the hydrogenation of benzaldehyde,
oxygen of PEG was coordinated to palladium atoms on the surface, and
possibly repulsion between benzyl alcohol and the electron-rich palla-
dium on the surface leads to inhibit further hydrogenation to form to-
luene, thus increased the selectivity of benzyl alcohol. Although the
selectivity of benzyl alcohol was achieved 100%, the highest benzal-
dehyde conversion was 84 % in liquid-phase hydrogenation. Therefore,
circumventing hydrogenolysis and phenyl ring reduction at high con-
version remains challenging in the hydrogenation of benzaldehyde to
benzyl alcohol [28,29].
In our previous work [30,31], we investigated the catalytic perfor-
mances of Pd/@-MexOy/AC(Me: Ti, Ce, La) for the hydrogenation of
phenol, we found that this Pd catalyst exhibited synergistic interactions
between the Pd nanoparticles and metal oxide. In this work, we pre-
pared a hybrid nano-structure Pd/@-ZrO2/AC catalyst. The Pd nano-
particles were anchored on the activated carbon (AC) which has a nano-
semiconductor layer. This Pd/@-ZrO2/AC catalyst exhibited strong
synergistic catalysis and showed excellent activity in the hydrogenation
of benzaldehyde to benzyl alcohol, much higher than that of Pd/ZrO2
and Pd/AC.
The preparation of metal oxide was the same as above, just not add
the AC.
Preparation of Pd/@-ZrO2/AC. It was adapted from the method of
our previous work [30,31]. As-prepared ZrO2/AC(0.475 g) was added
in deionized water(100 mL) and methanol(3 mL) mix solution, added
the H2PdCl4(0.4 mL, 0.012 g/mL) in it, ultrasonic dispersion for 30 min,
then ultraviolet lamp(220/15 W, Guangzhou Cnlight Optoelectronics
Technology Co., Ltd.) for 10 h. Finally, the sample was separated by
filtration, washed several times by distilled water up to pH = 7, dried in
a vacuum oven at 80℃ for 10 h. The preparation of Pd/@-La2O3/AC,
Pd/@-CeO2/AC, Pd/@-TiO2/AC, Pd/@-ZrO2/CNTs, Pd/@-TiO2, Pd/
CeO2, and Pd/La2O3 were the same as above, the only difference was to
ZrO2/AC with different supports.
Synthesis of Pd/AC or Pd/CNTs. Because the AC could not be ex-
cited and then generate electrons(e−) and holes(h+) under the UV ir-
radiation, so the Pd/AC or Pd/CNTs were prepared by liquid-phase
reduction. AC or CNTs was impregnated with an ethylene glycol solu-
tion containing H2PdCl4 and hydrazine hydrate, stirred at 80℃ for 4 h.
Then washed several times by distilled water and ethanol, dried in a
vacuum oven at 80℃ for 10 h.
2. Experimental
2.1. Catalyst preparation
The chemicals, if unspecified, were of analytical grade (A.R.) and
purchased from Tianjin Ke Miou Chemical Reagent Co., Ltd. The gases
were purchased from Xiangtan Gas Co., Ltd.
2.2. Catalyst characterization
Pretreatment of activated carbon with acid. A commercial activated
carbon (AC) made from coconut shells (Fujian Xinsen Carbon Co. Ltd.)
was pretreated with HNO3 (10 %) under refluxing at 60℃ for 2 h. Then
the AC was cooled to room temperature, washed to neutrality with
deionized water. Finally, the AC was dried at 120℃.
Preparation of ZrO2/AC. ZrOCl2·8H2O(1.453 g) dissolved in deio-
nized water(50 mL), while stirring, add the pre-processed AC(5 g) in it,
1.5 mol/L NH3·H2O was added dropwise until the pH of the resulting
solution was 10, the mixture was vigorously stirred during the addition,
after aging for 24 h, the precipitate was filtered and washed several
times by deionized water, and then dried at 100℃ followed by
X-ray diffraction (XRD) of samples was obtained on a Rigaku D/
max-II/2500 X-ray powder diffractometer, Cu Kα radiation was em-
ployed and the working voltage and current were 40 kV and 30 mA,
respectively. Transmission electron microscope(TEM) images were ob-
tained with a JEOL JEM-2100 F at an acceleration voltage of 200 kV. X-
ray photoelectron spectroscopy (XPS) was performing using ESCALAB
250Xi (Thermo) with Al Kα radiation. The specific surface areas of the
samples were calculated by the BET method by N2 adsorption-deso-
rption with
a
NOVA-2200e volumetric adsorption analyzer.
Temperature-programmed reduction by hydrogen (H2-TPR) was carried
out on Chem Bet 3000 analyzer. All samples (0.1 g) were pretreated in
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the flow of Ar at 250℃ for 1 h. Inductively coupled plasma-atomic
emission spectrometry (ICP-AES) data were obtained using an IntrepidⅡ
XSP (IRIS). Thermal Gravity Analysis (TGA) was performed on a TGA/
DSC1/1600 H T under a nitrogen atmosphere.
assigned to the formation of traces of Pd 3d5/2 and Pd 3d3/2, respec-
tively [35]. The measured values were slightly different from the
standard values(335 eV and 340.3 eV), indicated that there was a strong
metal-support interaction between nano-Pd and nano-ZrO2/AC. The
peaks at 336.8 eV correspond to the Pd 3d5/2 electronic states of Pd2+
species [36]. By calculating the integration areas of Pd° and Pd2+ of
Pd/@-ZrO2/AC catalyst, it can be estimated that the percentage of Pd°
and Pd2+ were 69.6 % and 30.4 %, respectively. The percents were
similar to those reported in the literature [37–39]. The results indicated
that Pd2+ could be reduced by photochemical route, the appearance of
Pd2+ species attributed to the electron transfer from Pd to ZrO2 due to
the strong interaction between Pd and ZrO2 [33]. The Pd 3d spectra of
Pd/ZrO2 also shown in Fig. 1(e), the percentage of Pd° and Pd2+ were
42.3 % and 57.7 %, respectively. Besides, Pd 3d5/2 for Pd2+ was
overlapped with Zr 3p3/2 in the XPS spectra, other researchers [33] also
observed this phenomenon. The Zr 3d spectra was shown in Fig. 1(f),
the peaks around 182.2 eV(Zr 3d5/2) and 184.5 eV(Zr 3d3/2) were as-
signed to the t-ZrO2 [40].
The TEM and HRTEM images of ZrO2/AC and Pd/@-ZrO2/AC were
shown in Fig. 2. For ZrO2/AC(Fig. 2a and b), the m-ZrO2 and t-ZrO2
occurred simultaneously. It was supportive of the XRD observations.
For Pd/@-ZrO2/AC(Fig. 2d–f), the distance of 0.223 nm(measured
crystal DigitalMicrograph) corresponding to the (111) plane of Pd.
Meanwhile, energy-dispersive X-ray spectroscopy (EDS) mapping
(Fig. 2g) confirmed the presence of elemental constituents with the as-
synthesized samples and showed that Pd and Zr were uniformly dis-
tributed. Combined with Fig. 2a, it was difficult to distinguish the Pd
nanoparticles and ZrO2 from Fig. 2d, so Fig. 2c was the size distribution
of Pd nanoparticles and ZrO2, the average size was around 2.6 nm.
2.3. Catalyst tests
The typical procedure for hydrogenation of benzaldehyde was as
follows: 0.05 g catalyst, 1.88 mmol benzaldehyde, and 20 mL solvent
were placed in an autoclave. The autoclave was purged with H2 to re-
move the air 3 times. Then the reaction was stirred at 800 rpm under
0.7 MPa H2 pressure until the temperature reached reaction tempera-
ture. After the end of the reaction, the autoclave was cooled to room
temperature, then analyzed using an Agilent 6890 N GC with an HP-5
(30m*0.32mm*0.25 μm) capillary column and FID detector. The tem-
perature of gasify room and detector were 250℃ and 280℃ respec-
tively. The air at a constant flow of 400 mL/min, the mixed gas H2 and
N2 flow of 30 mL/min. The initial column temperature was fixed at 70℃
for 1 min and then increased to 210℃ at a rate of 20℃/min. The final
temperature was maintained for 3 min. The conversion and selectivity
were calculated as follows:
Conversion of benzaldehyde(%) = (Moles of benzaldehyde converted)/
(Moles of benzaldehyde initially)×100%.0 %
Selectivity of benzyl alcohol(%) = (Moles of benzyl alcohol)/(Moles of
benzaldehyde converted)×100%.0 %
3. Results and discussion
3.1. Characterization
4. Hydrogenation of benzaldehyde
The relevant physicochemical parameters of the Pd/@-ZrO2/AC
catalyst were shown in Fig. 1. From Fig. 1(a), the ZrO2/AC and Pd/@
-ZrO2/AC samples displayed the typical diffraction peaks at 2Ө = 30.2°,
35.0°, 50.4° and 60.0°, which were readily assigned to tetragonal
ZrO2(t-ZrO2)(JCPD 03-0640), the peaks at 2Ө of 27.9° was assigned to
monoclinic ZrO2(m-ZrO2)(JCPD 01-0750). For Pd/@-ZrO2/AC, the
characteristic diffraction peaks at 2Ө = 40.2° and 46.6° corresponding
to (111) and (200) crystalline planes of Pd(JCPD 65–2867).
The TPR profiles were compiled in Fig. 1(b). The H2 consumption
peaks at low and high reduction temperatures were assigned to the
reduction of PdO having a weak and strong interaction with ZrO2
support [33]. For Pd/@-ZrO2/AC, the reduction peaks of PdO at above
183℃, but the reduction temperature of individual PdO was 50℃ [34].
It was demonstrated that the hybrid nano-structure catalyst exhibited
an interaction between nano-Pd and nano-ZrO2/AC. Zheng et al. [33]
studied H2-TPR of Pd/ZrO2 which obtained different calcination tem-
peratures of ZrO2(300, 500 and 600℃). They concluded that the in-
tensity of H2 consumption peak of Pd/ZrO2-500 at high reduction
temperature(∼110℃) was higher than that of Pd/ZrO2-300(∼95℃)
and Pd/ZrO2-600(∼100℃). Pd/ZrO2-500 exhibited a relatively high
strong metal-support interaction than other catalysts. Compared with
Pd/ZrO2-500, a shift towards higher temperature(183℃) was visualized
by Pd/@-ZrO2/AC(ZrO2/AC calcined at 500℃), which exhibited
stronger metal-support interaction of Pd/@-ZrO2/AC than that of Pd/
ZrO2-500.
The activity of Pd-based catalysts for the hydrogenation of benzal-
dehyde was then investigated. For our purpose, we first investigated the
effect of various solvents for the reaction. The corresponding conver-
sion and selectivity list in Table 1 and the reaction pathways were
shown in Scheme 2. When diethyl ether as a solvent, the benzaldehyde
conversion was only 60.1 %, and a large amount of toluene (24.0 %)
appeared in the product, the benzyl alcohol selectivity was only 70.3 %,
and totally hydrogenation product methylcyclohexane (5.7 %) has ap-
peared. The order of conversion of benzaldehyde→benzyl alcohol→
toluene is predominant, while the formation of side ethers was actually
inhibited, and (diethoxymethyl)benzene was not detected in the pro-
duct. Methanol and ethanol gave a higher conversion but with a low
selectivity toward benzyl alcohol. Because the benzaldehyde hydro-
genation product benzyl alcohol contains hydroxyl groups, and me-
thanol and ethanol also contained hydroxyl groups, they can be com-
bined with each other in a hydrogen-bonding manner, so that the
methoxy group and the ethoxy group participate in the reaction and
generate (ethoxymethyl)benzene and anisole. These observations were
also reported by several groups [7,16,41]. As we all know, the ben-
zaldehyde and benzyl alcohol were soluble in diethyl ether, methanol
and ethanol, and slightly soluble in water. When added benzaldehyde,
benzyl alcohol and organic solvents in the autoclave, the mixture so-
lution showed a single-phase, benzyl alcohol was not dissociation of the
active site, so it was further hydrogenation for benzyl alcohol. When
added benzaldehyde and benzyl alcohol in water, the mixture solution
showed two phases, we conclude that the hydrogenation of benzalde-
hyde in water with the Pd/@-ZrO2/AC catalyst may take place at the
interface of aqueous and organic phases, inhibited the further hydro-
genation of benzyl alcohol. Among the solvents tested, it seems that
water has exhibited the highest activity(100%) and benzyl alcohol se-
lectivity (98.1 %), and the use of water as a solvent complies with the
rules of green chemistry.
The N2 adsorption and desorption isotherms of Pd/@-ZrO2/AC and
ZrO2/AC were shown in Fig. 1(c), and these two samples displayed type
IV isotherms. Accordingly, the BET specific surface areas of AC, ZrO2/
AC, and Pd/@-ZrO2/AC were 1419 m2/g, 1183 m2/g, and 1162 m2/g,
respectively.
Fig. 1(d–f) showed the survey XPS spectrum and Pd 3d, Zr 3d
spectra of the Pd/@-ZrO2/AC catalyst. From Fig. 1(d), Pd, Zr, C, O was
detected in the sample. In Fig. 1(e), two signals were observed at values
of binding energy around 335.1 eV and 341.1 eV, which could be
The catalytic activities of these Pd catalysts in benzaldehyde hy-
drogenation were shown in Table 2. The ZrO2/AC support (Table 2,
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Fig. 1. XRD patterns (a) and H2-TPR profiles (b) of Pd/@-ZrO2/AC, ZrO2/AC and AC; N2 adsorption and desorption isotherms of Pd/@-ZrO2/AC and ZrO2/AC(c);
survey XPS spectrum (d) and Pd 3d (e) Zr 3d (f) spectra of Pd/@-ZrO2/AC and Pd/ZrO2.
Entry 2) had no catalytic activity. Pd/ZrO2 (Table 2, Entry 3) exhibited
the lowest activity with a benzaldehyde conversion of 23.3 %, Pd/AC
(Table 2, Entry 1) also gave an unsatisfactory conversion (29.7 %). It
was interesting to note that Pd/@-ZrO2/AC showed a remarkable
conversion of 100% (Table 2, Entry 5) only within 30 min. Extended the
reaction time to 60 min, the Pd/ZrO2 and Pd/AC obtained 51.6 % and
70 % conversion, respectively, which was still lower than that of Pd/@
-ZrO2/AC. This result indicated that the activity was based on two
major factors: (i) the special structure. The Pd/ZrO2 and Pd/AC were
the conventional catalysts which supported Pd on the support directly,
this structure has been studied for many years in the selective hydro-
genation, the activity was far from satisfactory, and thus researchers
made the oxide into a special morphology to increase the surface area
or added modifiers to improve the properties of the catalyst to improve
the catalytic performance. Obviously, just because of the special
structure of the Pd/@-ZrO2/AC improved the activity, Pd supported on
the ZrO2/AC could produce novel materials, which not only take ad-
vantage of AC with a high surface area but also have the properties that
the individual metal oxide does not have. (ii) The strong synergistic
effect in the Pd/@-ZrO2/AC catalyst. The turnover frequency(TOF,
TOF=(Moles of reactant)/((load of Pt × m(cat.)×Pt dispersion/
195.084)×(reaction time))) of Pd/@-ZrO2/AC(16439 h−1) was much
higher than Pd/ZrO2(6675 h−1) and Pd/AC(7264 h−1). This result in-
dicated that there was a strong synergistic effect between metal nano-
Pd and ZrO2/AC.
When the Pd loading decreased to 0.8 % in Pd/@-ZrO2/AC, 95.4 %
conversion of benzaldehyde was achieved in water within 50 min
(Table 2, Entry 8). When the Pd loading further decreased to 0.5 %, the
catalyst attained 96.0 % conversion within 70 min (Table 2, Entry 10).
Therefore, when decreased the loading of Pd, a high conversion of
benzaldehyde can also be achieved by prolonging the reaction time.
To further understand the synergistic effect of this hybrid nano-
structure catalyst, we investigated the hydrogenation of benzaldehyde
over Pd/@-MexOy/AC(MexOy: TiO2, CeO2, La2O3) catalysts. As can be
seen, the activity of Pd/@-MexOy/AC (Table 2, Entry 15, 18, 21) was
higher than that of Pd/MexOy (Table 2, Entry 11–13). It indicated that
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Fig. 2. TEM and HR-TEM images of ZrO2/AC (a–b) and Pd/@-ZrO2/AC (d–f); particle size distribution of Pd/@-ZrO2/AC(e); an enlarged view of the portion
containing Pd and ZrO2 in Fig. 2e(f); EDS maps revealing that the Pd and the Zr elements are homogeneously distributed in Pd/@-ZrO2/AC(g).
Pd supported on MexOy/AC could form the hybrid nano-structure cat-
alyst, which not only took advantage of the large specific surface area
and pore volume of the carbon material but also had a property not
possessed by a single metal oxide. The above results further confirmed
the synergistic catalysis of these Pd/@-MexOy/AC catalysts.
The content of metal oxide played an important role in the reaction.
When the metal oxide content increased from 5 to 10 wt.% (Table 2,
Entry 4–5, 14–15, 17–18, 20–21), the reaction went much faster. But
increased the content of metal oxide from 10 to 15 wt.%, the conversion
showed a downward trend. The group of Xie [30] reported that when
the content of metal oxide was less than a certain threshold and surface
area of the support was large enough, metal oxide would disperse on
the support at the state of the spontaneous mono-layer distribution.
When decreased the content of metal oxide, although the metal oxide
was still dispersed on the AC at the state of the spontaneous mono-layer
distribution, it was less than enough to cover the entire surface of the
AC, the Pd/@-5 %MexOy/AC consisted of major Pd/AC and slight Pd/@
-MexOy/AC. The structure of Pd/AC catalyst was supported Pd on the
AC directly, this structure(metal/support) has been studied for many
years, the activity was far from satisfactory. From Table 2, the Pd/AC
only obtained 29.7 % conversion. When increased the content of the
metal oxide to 15 wt.%, a small amount of metal oxide was dispersed on
Table 1
Hydrogenation of benzaldehyde over Pd/@-ZrO2/AC catalyst by different solvents.
Entry
Solvent
Conversion (%)
Selectivity(%)a
BA
T
MC
DMB/EMB
A
1
2
3
4
CH3CH2OCH2CH3
CH3OH
CH3CH2OH
H2O
60.1
83.9
99.4
100
70.3
21.9
5.8
24.0
33.7
71.5
1.9
5.7
12.6
7.1
–
–
–
15.6
–
–
31.8
–
–
98.1
Reaction conditions: catalyst(0.05 g), benzaldehyde(1.88 mmol), 30 min, 40℃, 0.7 MPa H2 pressure, solvent(20 mL).
a
Selectivity for benzyl alcohol(BA), toluene(T), methylcyclohexane(MC), (diethoxymethyl)benzene(DMB), (ethoxymethyl)benzene(EMB), anisole(A).
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Scheme 2. Reaction pathways of benzaldehyde hydrogenation in different
solvents.
Fig. 3. Pd 3d spectra of Pd/@-TiO2/AC, Pd/@-CeO2/AC and Pd/@-La2O3/AC.
the AC at the state of the multilayer distribution, the surface area also
decreased which might inhibit the adsorption of the reactants, resulting
in a decrease in conversion.
Table 3
Pd 3d XPS data and surface area of Pd/@-TiO2/AC, Pd/@-CeO2/AC and Pd/@
-La2O3/AC.
In addition, Table 2 also revealed the difference in catalytic activity
between Pd/@-ZrO2/AC, Pd/@-TiO2/AC, Pd/@-CeO2/AC and Pd/@
-La2O3/AC catalysts. The order of activity of the catalyst was Pd/@
-ZrO2/AC > Pd/@-TiO2/AC > Pd/@-La2O3/AC > Pd/@-CeO2/AC.
The specific surface areas of Pd/@-ZrO2/AC, Pd/@-TiO2/AC, Pd/@
-La2O3/AC and Pd/@-CeO2/AC were 1162 m2/g, 1188 m2/g, 1368 m2/
g and 1245 m2/g, respectively. This result indicated that it was not the
higher the specific surface area, the higher the activity. Then we cal-
culated the percentage of Pd° and Pd2+ of these catalysts according to
the XPS spectrum (Fig. 3, Table 3). It was known that the activation of
hydrogen molecules was usually by the Pd°, the electron-deficient Pd2+
species in the catalysts, which served as the electrophilic sites of ad-
sorption and activation of the C]O bond in the benzaldehyde
Sample
Pd0(eV)
Pd2+(eV)
Pd0/Pd2+(%) Surface area
(m2/g)
Pd/@-TiO2/AC
Pd/@-CeO2/AC
340.8 335.7 343.1 337.8 30.8/69.1
341.1 336.0 342.8 337.1 67.1/32.9
1180
1245
1368
Pd/@-La2O3/AC 340.9 336.0 343.2 337.9 59.5/40.5
molecules [20]. From Table 3, these Pd catalysts with different metal
oxide showed different electronic states of Pd. The Pd/@-TiO2/AC had
the highest percentage of Pd2+ (69.1 %), which can be assigned to more
electron transfer from Pd to TiO2. Although a high percentage of Pd2+
species adsorbed more benzaldehyde molecules, this didn't show high
activity. Actually, the activation of hydrogen molecules or the
Table 2
Hydrogenation of benzaldehyde over different catalysts.
Entry
Catalyst
Time (min)
Conversion (%)
Selectivity(%)a
BA
T
1
2
3
4
5
6
7
8
1 %Pd/AC
10 %ZrO2/AC
1 %Pd/ZrO2
30
30
30
30
30
30
30
50
30
70
60
60
60
60
60
60
60
60
60
60
60
60
30
60
30
30
29.7
–
23.3
44.6
100
99.1
–
100
0.9
–
0
1 %Pd/@-5 %ZrO2/AC
1 %Pd/@-10 %ZrO2/AC
1 %Pd/@-15 %ZrO2/AC
0.8 %Pd/@-10 %ZrO2/AC
0.8 %Pd/@-10 %ZrO2/AC
0.5 %Pd/@-10 %ZrO2/AC
0.5 %Pd/@-10 %ZrO2/AC
1 %Pd/TiO2
99.2
98.1
98.3
98.6
98.8
99.1
98.4
99.5
99.7
100
0.8
1.9
1.7
1.4
1.2
0.9
1.6
0.5
0.3
0
95.0
76.5
95.4
68.1
96.0
43.2
55.6
49.0
38.1
95.9
80.1
50.5
87.3
82.1
37.7
89.4
85.1
42.0
98.6
87.2
100
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23b
24b
25
26
1 %Pd/CeO2
1 %Pd/La2O3
1 %Pd/@-5 %TiO2/AC
1 %Pd/@-10 %TiO2/AC
1 %Pd/@-15 %TiO2/AC
1 %Pd/@-5 %CeO2/AC
1 %Pd/@-10 %CeO2/AC
1 %Pd/@-15 %CeO2/AC
1 %Pd/@-5 %La2O3/AC
1 %Pd/@-10 %La2O3/AC
1 %Pd/@-15 %La2O3/AC
1 %Pd/@-10 %ZrO2/AC
1 %Pd/@-10 %ZrO2/AC
1 %Pd/CNTs
100
0
98.0
98.3
99.1
98.6
98.8
100
99.4
99.8
100
2.0
1.7
0.9
1.4
1.2
0
0.6
0.2
0
99.1
99.5
97.2
0.9
0.5
2.8
1 %Pd/@-10 %ZrO2/CNTs
Reaction conditions: catalyst(0.05 g), benzaldehyde(1.88 mmol), 40℃, 0.7 MPa H2 pressure, H2O(20 mL).
a
Selectivity for benzyl alcohol(BA), toluene(T).
Second run to test the reusability of Pd/@-ZrO2/AC.
b
6

Y. Zhang, et al.
CatalysisTodayxxx(xxxx)xxx–xxx
% conversion and 99.1 % benzyl alcohol selectivity (Table 2, Entry 24).
The Pd content of used Pd/@-ZrO2/AC catalyst was about 0.9 wt.%,
almost the same as that of the fresh Pd/@-ZrO2/AC catalyst(0.92 wt.%).
From Figure S1a, for the fresh and used catalyst, the weight losses oc-
curred at 30-100℃, which was due to the desorption of physically ab-
sorbed water. For the used catalyst, a slight weight loss in the range of
150-300℃ corresponded to the decomposition of organics. Combined
with N2 adsorption-desorption analysis, the specific surface area of
catalyst was reduced from 1162 m2/g to 925 m2/g and pore volume is
decreased from 0.6 to 0.5 cm3/g. TEM images of fresh and used cata-
lysts were shown in Figure S2(a–b). Compared with fresh catalyst, the
Pd nanoparticles of used Pd/@-ZrO2/AC catalyst was obviously ag-
glomerated. This result indicates that the reason for the deactivation
could be mainly ascribed to the pore blockage with organic matters
including benzaldehyde and benzyl alcohol, and the agglomeration of
Pd nanoparticles.
According to the literature report [43,44], the support with regular
pore structure may maintain the structure of the catalyst to keep its
stability, so we used the CNTs as support to investigate the stability of
this hybrid nano-structure Pd catalyst, obtained Pd/@-ZrO2/CNTs cat-
alyst. This catalyst also exhibited synergistic catalysis, the order of the
activity was Pd/@-ZrO2/CNTs(100%) > Pd/CNTs(87.2%) > Pd/ZrO2
(23.3%)(40℃, 30 min, 0.7 MPa). The result of catalytic hydrogenation
of benzaldehyde was shown in Fig. 6. To our delight, the catalyst
showed no distinct loss in activity or selectivity after second time cy-
cles. When the catalyst was subjected to the fifth cycles, it still main-
tained a higher activity (62 %) than that of Pd/ZrO2 (23.3 %). Although
this Pd/@-ZrO2/CNTs catalyst can avoid organic blocking pores(Figure
S1b, Figure S3), Pd nanoparticles still aggregated after five cycles
(Figure S2c-d). These results indicate that in order to avoid structural
changes or pore-clogging after multiple reuses of the catalyst, it is ne-
cessary to develop a catalyst for immobilizing Pd nanoparticles on a
support having a regular pore structure and a high degree of stability
for the process. Based on the above discussion, whether AC or CNTs as a
carrier, these hybrid nano-structure catalysts can exhibit a synergistic
effect, and we believe our strategy offers an attractive opportunity for
selectivity hydrogenation.
Fig. 4. The acidity properties of the Pd/@-ZrO2/AC, Pd/@-TiO2/AC, Pd/@
-CeO2/AC and Pd/@-La2O3/AC catalyst by NH3-TPD.
adsorption of benzaldehyde molecules may be "just right" at some point
of the relative ratio of the Pd species, so that the catalyst obtains the
highest activity.
As we all know, the acidity of the catalyst may affect the catalytic
activity, the NH3-TPD (Fig. 4, Table 4) measurements were carried out
to achieve the acidity of the Pd/@-MexOy/AC catalysts. From Table 4,
the total amount of acidity sites is in the order of Pd/@-ZrO2/AC >
Pd/@-La2O3/AC > Pd/@-CeO2/AC > Pd/@-TiO2/AC.
Obviously,
the acidity was not the major reason to influence the activity.
It has been reported that both the support and the surface metal
atoms may be involved in determining the performance of catalysts
[42]. Usually, the high dispersed noble metal catalyst often exhibits
high catalytic performance. To learn more in-depth on the difference
between the catalysts, the Pd/@-MexOy/AC catalyst was characterized
by TEM and HR-TEM (Fig. 5). As can be seen, it was difficult to dis-
tinguish the Pd nanoparticles and metal oxide from Figs. 2d and 5 a, e, i,
so we gave the average particle sizes of Pd and metal oxide, the order
was
Pd/@-ZrO2/AC(2.6 nm) < Pd/@-TiO2/AC(3.4 nm) < Pd/@
-La2O3/AC(8.7 nm) < Pd/@-CeO2/AC(11.9 nm). According to the CO
pulse chemisorption, the order of the Pd nanoparticle size was Pd/@
-ZrO2/AC(1.8 nm) < Pd/@-TiO2/AC(2.3 nm) < Pd/@-La2O3/
5. Conclusions
AC(4.3 nm) < Pd/@-CeO2/AC(8.5 nm), the order of the Pt dispersion
was Pd/@-ZrO2/AC (61.0 %) > Pd/@-TiO2/AC (49.4 %) > Pd/@
-La2O3/AC (25.9 %) > Pd/@-CeO2/AC (13.3%).
In summary, a hybrid nano-structure Pd/@-ZrO2/AC catalyst has
been prepared by a photochemical route. This catalyst exhibited high
activity(100% conversion, > 98% selectivity) for the hydrogenation of
benzaldehyde to benzyl alcohol at 40℃ using water as a clean solvent.
The high catalytic performance was attributed to the special structure
of the catalyst, and the synergistic effect between Pd nanoparticles and
ZrO2/AC, which leads to a well-dispersed of Pd coated on the ZrO2/AC
surface. And the Pd/@-TiO2/AC, Pd/@-CeO2/AC, and Pd/-La2O3/AC
catalysts further confirmed the synergistic effect in this hybrid nano-
structure for this reaction. After the second cycle, the Pd/@-ZrO2/AC
catalyst obtained 98.6 % conversion and 99.1 % benzyl alcohol se-
lectivity at 40℃ when extended the reaction time to 60 min. When
using CNTs as a support, the Pd/@-ZrO2/CNTs catalyst showed no
distinct loss in activity (98 % conversion) or selectivity (100%) after
second time cycles at 40℃ within 30 min. Thus, the combination of
nano-metals and nano-semiconductors/porous materials forms a hybrid
nano-structure catalyst with synergistic effects that are expected to be
used in a series of selective hydrogenation of heterogeneously catalyzed
reactions.
Table 2 shown that the order of activity of the catalyst was Pd/@
-ZrO2/AC > Pd/@-TiO2/AC > Pd/@-La2O3/AC > Pd/@-CeO2/AC.
The above results demonstrated that the activity of this Pd catalysts was
mainly affected by the particle size and Pt dispersion. The smaller the
particle size, the higher Pt dispersion and the higher the activity.
After the reaction, the used catalyst was washed by deionized water
(500 mL), then dried in a vacuum oven at 80℃ for 10 h. We investigated
the activity of the used Pd/@-ZrO2/AC catalyst under the same reaction
condition. The used catalyst only obtained 42.0 % conversion after the
second cycle (Table 2, Entry 23), extended to 60 min, it obtained 98.6
Table 4
The acidic properties of the Pd/@-ZrO2/AC catalyst by NH3-TPD.
Catalyst
250-500℃
> 500℃
Total nNH3
(μmol/g)
Peak
temperature
(℃)
nNH3
(μmol/
g)
Peak
temperature
(℃)
nNH3
(μmol/
g)
Acknowledgements
Pd/@-ZrO2/AC
Pd/@-TiO2/AC
Pd/@-CeO2/AC
361.8
425.7
423.6
793.4
713.5
1112.1
726.3
696.8
750.9
703.0
696.8
2863.5
1962.7
1633.5
2330.9
3656.9
2676.2
2745.6
3057.2
This work was supported by the Natural Science Foundation of
China (21676227), the Key Project of Hunan Provincial Natural Science
Foundation of China (09JJ3021).
Pd/@-La2O3/AC 424.5
7

Y. Zhang, et al.
CatalysisTodayxxx(xxxx)xxx–xxx
Fig. 5. TEM and HR-TEM images and particle size distributions of Pd/@-TiO2/AC (a–d); Pd/@-CeO2/AC (e–h); Pd/@-La2O3/AC (i–l).
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9